A crank device is known in the art as a means for performing power conversion between reciprocating motion and rotational motion in an internal combustion engine or a compressor. It is noted, however, that such a crank device presents loss in efficiency of power conversion, since a connecting rod coupling a piston and a crankshaft is always inclined relative to the cylinder axis except when the connecting rod is at the top dead center or the bottom dead center of the piston. It is also noted that a phenomenon called "piston slap" is caused in which the piston rubs against the inner wall of the cylinder due to a lateral component of force applied from the connecting rod to the piston as a reaction force. This causes vibration, noise and wearing.
It is noted, however, that employment of prior art crank device is unavoidable when it is intended to drive a driven machine including a movable body reciprocating in linear fashion, such as a piston of a compressor, using an internal combustion engine as a power source. FIG. 4 illustrates three types of coupling mechanisms between an internal combustion engine E and a compressor C. In FIGS. 4(a) and (b), each of the internal combustion engine E and the compressor C includes a crank device. In the coupling mechanism shown in FIG. 4(a), a rotational motion is transmitted by means of pulleys 50a and 50b and a belt 51. In the coupling mechanism shown in FIG. 4(b), a rotational motion is transmitted by means of a coupling 52. In the coupling mechanism shown in FIG. 4(c), a common crank device 53 is disposed at a position where the cylinder axes of the engine E and the compressor C intersect with each other. In this case, the internal combustion engine E and the compressor C share a common crank pin 54.
The above-mentioned three coupling mechanisms all present significant loss during power conversion, since they employ a crank device.
In order to eliminate the disadvantages in prior art crank device mentioned above, the inventors have proposed a new crank device based on a new concept (Japanese Patent Application No. 6-121776 (Hei-6-121776). According to the new crank device, a planetary gear mechanism is interposed between a connecting rod and a crankshaft, so as to allow lateral amplitude caused at the lower end of the connecting rod in response to rotation of crankshaft to be accommodated by the planetary gear mechanism. By this, the connecting rod is always moved up and down along the cylinder axes, without causing any rocking motions, whereby efficiency in power conversion may be increased and undesirable phenomenon of piston slap may be eliminated.